Financial Assets and Liabilities - Gross Obligations For Securities Lending Transactions (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Fair value measurements measured on a recurring basis
Remaining contractual maturity for repurchase agreements		$ 222,203
Remaining contractual maturity for securities lending transactions	$ 423,672
Overnight and Continuous
Fair value measurements measured on a recurring basis
Remaining contractual maturity for repurchase agreements		222,203
Remaining contractual maturity for securities lending transactions	423,672
Equity securities
Fair value measurements measured on a recurring basis
Remaining contractual maturity for repurchase agreements		222,203
Remaining contractual maturity for securities lending transactions	423,672
Equity securities | Overnight and Continuous
Fair value measurements measured on a recurring basis
Remaining contractual maturity for repurchase agreements		$ 222,203
Remaining contractual maturity for securities lending transactions	$ 423,672